Switch devices based on conductive liquids have been known since the 19th century. Recently, electrically-controlled, highly-miniaturized conductive liquid-based switches have been proposed. Such switches can be fabricated in a semiconductor substrate, and therefore can be integrated with other electrical devices fabricated in the substrate. Such switches have the advantage that they provide a substantially higher isolation between the control signal and the switched circuit than switch devices based on semiconductor devices.
Published Japanese Patent Application No. S47-21645 discloses an example of a switch device for electrically switching solid electrodes by means of a conductive liquid. In this switch device, a conductive liquid such as mercury is movably disposed inside a cylinder. The switch device is designed so that the conductive liquid is moved to one side by a pressure differential in a gas provided on both sides of the conductive liquid. When the conductive liquid moves, it touches electrodes that extend into the interior of the cylinder and forms an electrical connection between the electrodes. A drawback to this structure, however, is that the electrical connection characteristics of the switch device deteriorate as a result of the surfaces of the electrodes being modified over time by intermittent contact with the conductive liquid.
U.S. Pat. No. 6,323,447, assigned to the assignees of this disclosure and, for the United States, incorporated herein by reference, discloses a switch device that solves the above-mentioned problem. In this switch device, the electrical path is selectively changed from a connected state to a disconnected state by a conductive liquid such as mercury. However, the electrodes remain in constant contact with the conductive liquid, and the connected or disconnected state of the electrical path is determined by whether the conductive liquid exists as a single entity (connected state) or is separated into two discontinuous entities (disconnected state). This eliminates the problem of poor connections that was a disadvantage of the switch device disclosed in published Japanese Patent Application No. S47-21645.
The switch device described in U.S. Pat. No. 6,323,447 is composed of an elongate passage filled with a conductive liquid and having electrodes located at its ends, a first cavity filled with non-conductive fluid and connected to approximately the mid-point of the passage by a single channel, a second cavity filled with non-conductive fluid and connected to near the ends of the passage by two channels. A heater is located in each cavity.
The heater in the first cavity is activated to switch the switch device to its OFF state. Heat generated by the heater causes the non-conductive fluid in the cavity to expand. The excess volume of the non-conductive fluid passes though the single channel to the passage where it forms a gap in the conductive liquid. The gap filled with the non-conductive fluid electrically insulates the electrodes from one another. The conductive liquid displaced by the non-conductive fluid enters the channels at the ends of the passage.
The heater in the second cavity is activated to switch the switch device to its ON state. Heat generated by the heater causes the non-conductive fluid in the cavity to expand. The excess volume of the non-conductive fluid passes though the two channels to displace the conductive liquid from the channels. The conductive liquid returning to the passage displaces the non-conductive fluid from the gap and the conductive liquid returns to its continuous state. In this state, the conductive liquid electrically connects the electrodes.
Some embodiments of the switch device described in U.S. Pat. No. 6,323,447 include latching structures located in the channels connecting the cavities to the passage. The latching structures hold the switch device in the switching state to which it has been switched after the respective heater has been de-energized. The latching structures require the conductive liquid to enter the channels, which have somewhat smaller cross-sectional dimensions than the passage. This increases both the energy required to operate the switch and the time required to change the switching state of the switch.
Moreover, the latching structures may provide inadequate latching reliability for some applications. A substantial amount of the conductive liquid connects each latching structure to the respective surface of the conductive liquid. The conductive liquid connecting the latching structure to the surface is not fully bounded. A stimulus, such as vibration or a temperature change, can therefore cause the form of the conductive liquid to change to one that changes the switching state of the switch device.
Published International Patent Application No. WO 01/46975, assigned to the assignees of this disclosure and, for the United States, incorporated herein by reference, discloses a switch device in which the conductive liquid is confined to the passage. This decreases both the energy required to operate the switch and the time required to change the switching state of the switch compared with the switch device shown disclosed in U.S. Pat. No. 6,323,447. FIGS. 1A and 1B show an example 10 of the conductive liquid-based switch device disclosed in published International Patent Application No. WO 01/46975. Switch device 10 is composed of elongate passage 12, chambers 14 and 16, channels 18 and 20, non-conductive fluid 22 and 24, conductive liquid 26, electrodes 31 and 32 and heaters 50 and 52. Electrodes 30, 31 and 32 are disposed along the length of passage 12. Conductive liquid 26 is located in the passage and has a volume less than that of the passage so that the conductive liquid only partially fills the passage. The conductive liquid therefore exists as a number of conductive liquid portions 40, 41 and 42.
Channel 18 extends from cavity 14 to passage 12. Channel 20 extends from cavity 16 to the passage. The channels are offset from one another along the length of the passage and are located between electrode 30 and electrode 31 and between electrode 31 and electrode 32, respectively. Cavities 14 and 16 and channels 18 and 20 are filled with non-conductive fluid 22 and 24, respectively. Heaters 50 and 52 are located in cavities 14 and 16, respectively, for regulating the internal pressure of the non-conductive fluid in the cavities. Channels 18 and 20 transfer the non-conductive fluid in cavities 14 and 16, respectively, to and from passage 12.
The switching operation of switch device 10 is as follows. In the initial switching state shown in FIG. 1A, heater 50 is energized and heater 52 is not energized. Conductive liquid portions 41 and 42 are joined together to form conductive liquid portion 41, 42. Conductive liquid portion 41, 42 is separated from conductive liquid portion 40 by non-conductive fluid 22. Thus, conductive liquid portion 41, 42 electrically connects electrode 31 to electrode 32, but non-conductive fluid 22 between conductive liquid portion 41, 42 and conductive liquid portion 40 electrically insulates electrode 30 from electrode 31.
Switch device 10 switches to the switching state shown in FIG. 1B when heater 50 is de-energized and heater 52 is energized. Heat generated by heater 52 causes non-conductive fluid 24 in cavity 16 to expand. Non-conductive fluid 24 passes through channel 20 and enters passage 12. In the passage, non-conductive fluid 24 forms a gap in conductive liquid portion 41, 42 (FIG. 1A). The gap separates conductive liquid portion 41, 42 into non-contiguous conductive liquid portions 41 and 42. Separation of conductive liquid portion 41, 42 into conductive liquid portions 41 and 42 expels non-conductive fluid 22 from the gap between conductive liquid portions 40 and 41. This allows conductive liquid portions 40 and 41 to unite to form conductive liquid portion 40, 41. Conductive liquid portion 40, 41 electrically connects electrode 30 to electrode 31. Non-conductive fluid 22 in the gap between conductive liquid portion 42 and conductive liquid portion 40, 41 electrically insulates electrode 31 from electrode 32. Switch device 10 stays in the switching state shown in FIG. 1B for as long as heater 52 is energized.
Switch device 10 returns to the switching state shown in FIG. 1A when heater 52 is de-energized and heater 50 is energized. Heat generated by heater 50 causes non-conductive fluid 22 in cavity 14 to expand. Non-conductive fluid 22 passes though channel 18 and enters passage 12. In the passage, non-conductive fluid 22 forms a gap in conductive liquid portion 40, 41 (FIG. 1B). The gap separates conductive liquid portion 40, 41 into non-contiguous conductive liquid portions 40 and 41. Separation of conductive liquid portion 40, 41 expels non-conductive fluid 24 from the gap between conductive liquid portions 41 and 42. This allows conductive liquid portions 41 and 42 to unite to form conductive liquid portion 41, 42. Conductive liquid portion 41, 42 electrically connects electrode 32 to electrode 31. Non-conductive fluid 22 electrically insulates electrode 30 from electrode 31.
Switch device 10 is non-latching. Heater 50 must be continuously energized to hold the switch device in the switching state shown in FIG. 1A and heater 52 must be continuously energized to hold the switch device in the switching state shown in FIG. 1B. De-energizing heater 50 after switching the switch device to the switching state shown in FIG. 1A would incur the risk that the resulting contraction of non-conductive fluid 22 would allow conductive liquid portions 40 and 41, 42 to unite to form an electrical connection between electrodes 30 and 31. The contraction of non-conductive fluid 22 would incur the additional risk that conductive liquid portion 41, 42 would fragment into conductive liquid portions 41 and 42 to break the electrical connection between electrodes 31 and 32. In other words, there is the risk that, on de-energizing heater 50, switch device 10 would spontaneously revert to the switching state shown in FIG. 1B or to an indeterminate switching state. Corresponding risks would exist if heater 52 were de-energized off after switching switch device 10 to the switching state shown in FIG. 1B.
Thus, energy has to be continuously expended to maintain the switch device 10 in the switching states to which it has been switched. This is undesirable in terms of expense, energy conservation and energy dissipation. Attempting to save energy by de-energizing the heaters after switching incurs the risk of the switch device reverting to the other switching state or to an indeterminate state. In many applications such risks are unacceptable.
What is needed, therefore, is a conductive liquid-based switch device that requires a relatively small input of energy to change it rapidly from one switching state to the other. What is also needed is a conductive liquid-based switch device that is latching in each of its switching states so that it only needs an input of energy to switch it from switching state to another. Finally, what is needed is a conductive liquid-based switch device that reliably maintains the switching state to which it has been switched without a continuous input of energy.
The invention provides a latching switch device that comprises a passage, a first cavity, a second cavity, a channel extending from each cavity to the passage, non-conductive fluid located the cavities, conductive liquid located in the passage, a first electrode, a second electrode and a latching structure associated with each channel. The passage is elongate. The channels are spatially separated from one another along the length of the passage. The electrodes are in electrical contact with the conductive liquid and are located on opposite sides of one of the channels. The conductive liquid includes free surfaces. Each latching structure includes energy barriers located in the passage on opposite sides of the channel. The energy barriers interact with the free surfaces of the conductive liquid to hold the free surfaces apart from one another.
The latching structure allows the heater to be de-energized after changing the switching state of the switch device without the risk of the switch device spontaneously reverting to the other switching state or to an indeterminate switching state. When the heater is de-energized, the non-conductive fluid contracts. However, the latching structure and, specifically, the energy barriers, hold the surfaces of the conductive liquid apart. As a result, the switch device reliably maintains the switching state to which it was switched when the heater was energized. The latching structure ensures that the switch device can only be switched to its other switching state by energizing the other heater.
Energizing the heaters only to change the switching state of the switch device and not to maintain the switch device in the corresponding switching state substantially reduces the power consumption of the switch device compared with conventional liquid conductor-based switch devices.
The latching structures interact directly with the free surfaces of the conductive liquid portions to keep the free surfaces apart and maintain the switch device in the switching state to which it has been switched. The latching structure is not connected to the free surfaces by a thread of conductive liquid whose form can change and allow the free surfaces to move into contact with one another. Also, one end of each conductive liquid portion is bounded by an end of passage and the other end of the conductive liquid portion is bounded by one of the energy barriers. Since the conductive liquid portion is bounded at both of its ends, its ability to change its form and open the electrical connection between the electrodes in contact with it is substantially reduced.
The invention also provides a latching switch device that comprises a passage, a first cavity, a second cavity, a channel extending from each cavity to the passage, non-conductive fluid located the cavities, conductive liquid located in the passage, a first electrode and a second electrode. The passage is elongate. The channels are spatially separated from one another along the length of the passage. The electrodes are in electrical contact with the conductive liquid and are located on opposite sides of one of the channels. The passage includes a latching structure associated with each channel. Each latching structure includes a first low surface energy portion, a high surface energy portion and a second low surface energy portion arranged in tandem along part of the length of the passage. The high surface energy portion is located at the channel. The low surface energy portions are structured to reduce the surface energy of the conductive liquid relative to the surface energy of the conductive liquid in the high surface energy portion.
In the latching structure associated with each channel, the low surface energy portions and the high surface energy portion collectively form two energy barriers located adjacent, and on opposite sides of, the channel. When the heater associated with the channel is energized to switch the switching state of the switch device, non-conductive fluid is output from the channel and divides the conductive liquid portion adjacent the channel into two smaller conductive liquid portions. This forms a free surface on each of the conductive liquid portions. The non-conductive fluid moves the free surfaces away from the channel and across the energy barriers.